Linear beam tube employing a shielded beam focus solenoid having a reentrant cathode pole piece



G. v. MIRAM 3,475,644 YING A SHIELDED BEAM FOCUS SO NG A REENTRANTCATHODE POLE PIECE Oct. 28. 1969 LENOID LINEAR BEAM TUBE EMPLO HAVIFiled April 14, 1967 2 sheets-Sheet 1 FIG. 2 PRIOR ART FiG. I PRIOR ARTPEG. 4 PRIOR ART FIG. 3 PRIOR ART INVENTOR. GEORGE v. MIRAM azzwATTORNEY Oct. 28. 1969 LINEAR BEAM TUBE EMPLOYING A SHIELDED BEAM FOCUSSOLENOID MIRAM 3,475,644

HAVING A REENTRANT CATHODE POLE PIECE F1191 April 14, 1967 2Sheets-Sheet 2 INVENTOR.

GEORGE V. MIRAM Mia/e: ATTORNEY US. Cl. 315-35 5 Claims ABSTRACT OF THEDISCLOSURE A 100 kw. average power X-band tube is disclosed whichemploys a highly convergent linear beam of electrons focused by means ofa shielded solenoid. The solenoid has a cathode pole piece whichincludes a reentrant central portion. The cathode emitter is located inthe plane of the mouth of the reentrant portion. Electrons emitted fromthe cathode are focused into a highly convergent beam, i.e., areaconvergence greater than thirty, such as 50:1, 70:1 or 100:1 andprojected through a narrow throat portion of the reentrant pole piecestructure into a uniform region of beam focusing magnetic field.

The solenoid includes a cylindrical magnetic yoke which surrounds thesolenoid and serves to produce an axially symmetric convergent magneticfield of the precise and proper convergence between the cathode emitterand the place where the beam reaches its minimum diameter to eliminateor minimize beam scalloping;

In one embodiment, a modulating anode electrode is also disposed withinthe reentrant cathode pole piece structure. A tubular portion of themodulating anode projects into the narrow throat of the magnetic polepiece and is insulated and spaced from the pole piece which operates atmain anode potential.

Description of the prior art Heretofore, 100 kw. average power X-bandklystron tubes have been built which employed a split solenoid with arectangular open-yoke having a pair of mutually opposed reentrant polepieces. The magnetic beam focusing structure was similar to thatdescribed in US. Patent 2,928,972, issued Mar. 15, 1960 and assigned tothe same assignee as the present invention. In this prior tube, therewere 5 axially spaced stagger tuned cavity resonators in a magnetic gaphaving a length of about 3.5". The tube had a bandwidth of about 0.5%.It was desired to provide an instantaneous bandwidth of 4%. In order toprovide 4% instantaneous bandwidth, the output circuit would require aplurality of coupled cavities thereby extending the length of themagnetic circuit from 3.5" to between 5.5" and 7" depending upon thetype of buncher and output circuits used. Such a long magnetic gap ofrelatively intense field as of 3.5 to 4.5 kg. coupled with therequirement of a highly convergent beam, i.e., area convergence greaterthan 30 and very low beam interception, i.e., less than 1% andreasonable radio frequency efficiencies of 45% at X-band mitigateagainst the use of a split-solenoid open-yoke electromagnet. Moreparticularly, the splitsolenoid produces a dip in the beam focusingfield in the gap of the magnet. This dip tends to cause beam blow-up andto produce amplification of beam scalloping. Also beam scalloping isaggravated by the open-yoke structure since the convergent magneticfield in the electron gun is not always symmetric about thecircumference of the beam due to saturation of the pole pieces and thenon-symmetric nature of the open-yoke structure.

Cylindrical closed-yoke solenoidal electromagnets have also been usedfor focusing linear beam tubes. However,

United States atent 0 in such prior magnetic circuits the cathode isgenerally axially spaced outside of the pole plate disposed adjacent thecathode (cathode pole plate). A tubular magnetic field shaping member isemployed which projects axially from the cathode pole plate toward andaround the cathode emitter. The tubular field shaping member serves toshield the cathode from the leakage flux from the magnet. However, suchmagnetic circuits fail to provide the necessary degree and rate ofmagnetic field convergence in the electron gun, and do not provide thenecessary degree of shielding.

Summary of the present invention The principal object of the presentinvention is the provision of an improved beam focusing magnetic circuitfor linear beam tubes.

One feature of the present invention is the provision of a beam focusingmagnetic circuit for a high convergence linear beam tube including asolenoid surrounded by an enclosing magnetic yoke having a reentrantcathode pole structure at one end and a cathode emitter disposed nearthe plane of the mouth of the reentrant portion for projecting a beam ofelectrons through a throat portion of the pole into a uniform beam focusmagnetic field, whereby the high convergence beam is focused over asubstantial gap length without beam blow-up or scalloping.

Another feature of the present invention is the same as the precedingfeature including the provision of a modulating anode electrode disposedwithin the reentrant portion of the cathode pole piece and having atubular section extending into the throat of the reentrant pole piecestructure in spaced relation therefrom, whereby the beam is modulated asdesired.

Another feature of the present invention is the same as any one or moreof the preceding features wherein the region of uniform magnetic fieldin the gap of the electromagnet is constant to within 10% of its peakintensity, whereby beam blow-up and scalloping are minimized.

Other features and advantages of the present invention will becomeapparent upon a perusal of the following specification taken inconnection with the accompanying drawings wherein:

Brief description of the drawings FIGURE 1 is a longitudinal view,partly in section, of a prior art beam focusing electromagnet,

FIGURE 2 is a view of the structure of FIGURE 1, partly broken away,taken along line 22 in direction of the arrows,

FIGURE 3 is a plot of magnetic field intensity versus axial distancealong the centerline of the structure of FIGURE 1,

FIGURE 4 is a longitudinal sectional view, partly schematic, of a priorart beam focus solenoid,

FIGURE 5 is a schematic line diagram of a beam profile showing beamscalloping and blow-up,

FIGURE 6 is a longitudinal sectional view of a microwave tube and itsbeam focusing solenoid incorporating features of the present invention,

FIGURE 7 is a reduced sectional view of the structure of FIGURE 6 takenalong line 77 in the direction of the arrows,

FIGURE 8 is a schematic diagram of the gun portion of the structure ofFIGURE 6 delineated by line 88 with a plot of beam profile and magneticfield versus arial and radial distances superimposed thereon, and

FIGURE 9 is an enlarged sectional view of an alternative structure tothat portion of the structure of FIGURE 6 delineated by line 88.

Description of the preferred embodiments Referring now to FIGURES 1 and2 there is shown a prior art open-yoke split-solenoid beam focusingelectromagnet as used for narrow band X-band high power klystron tubes.The magnetic circuit includes a rectangular yoke 1, a soft iron, havingtwo coaxially disposed hollow reentrant pole pieces 2 and 3, as of softiron. A magnetic gap 4 is defined between the opposed inwardly taperedends of pole pieces 2 and 3. A solenoid which is split into two seriesconnected sections 5 and 6 is wound over the pole pieces 2 and 3.

An X-band 5 cavity klystron amplifier 7, only partially shown, iscoaxially disposed of the hollow pole pieces 2 and 3 and of the gap 4.The beam focusing magnet produces a magnetic field on the axis of thebeam within the klystron 7 as shown in FIGURE 3. The cathode emitter 8of the tube 7, schematically shown in FIGURE 1, is disposed in theconvergent region of the magnetic field. The magnetic pole structure 2is shaped such that the magnetic flux lines in the electron gun regionare matched to the trajectories that the electrons will follow underelectrostatic forces alone. The convergence of the magnetic field in theelectron gun region is determined by finding the proper size andposition of the hole in the cathode pole piece 2 so that the magneticflux leaking through it from the main magnet gap 4 will match thedesired electron trajectories.

Due to the split nature of the solenoid 5 and 6, the beam focusing fieldH has a substantial dip in the center of the gap 4. The gap 4 is onlyabout 3.5" long and, thus, the magnetic field clips from a high value toa lower value in a relatively short distance. This abrupt change in themagnetic field tends to cause beam blow-up and to amplify beamscalloping, as indicated by line 11 of FIGURE 5. If the dip in fieldintensity were not present, the beam would not blow up and would followthe profile trajectory indicated by line 12 of FIGURE 5. Also, the beamscalloping would be damped by the uniform field. The beam scalloping iscaused by departure of the magnetic field from the proper shape in thegun region. Scalloping is encountered in the open-yoke design of FIGURES1 and 2 due to the lack of symmetry of the field about the circumferenceof the beam due to the lack of symmetry of the open-yoke structure 1 andmagnetic saturation effects in the pole structure.

Referring now to FIGURE 4 there is shown another prior art magnetic beamfocusing electromagnet 13. In this design, longer magnetic gaps areobtained with more uniform beam focusing magnetic fields, as indicatedby the dotted line 14 of FIGURE 3. The electromagnet 13 includes asolenoid 15 surrounded by a cylindrical magnetic yoke 16 which is closedat its ends by a pair of annular pole plates 17 and 18, as of iron. Themagnetic field is shaped in the region of the electron gun 8 by means oftubular iron projection 21 which projects from the cathode pole plate 17toward and around the cathode emitter 8. A non-magnetic anode insert 22forms the anode electrode. While the magnet 13 provides a uniform fieldover a relatively long gap it provides insufficient convergence for highconvergence electron guns, i.e., area convergence greater than 30 to 1and preferably in the range of 50:1 to- 100:1.

Referring now to FIGURES 6-8, there is shown a linear beam X-band tubeincorporating features of the present invention. The magnetic circuitincludes a cylindrical solenoid 31 separated into three closely spacedsegments separated by water-cooled plates 32, as of copper. The plates32, as of 0.250" thick, include annular coolant channels 33 connected toa source of liquid coolant via pipes 34.

A cylindrical magnetic yoke 35, as of 0.625" thick soft iron, with anoutside diameter of 16 and a length of 8.25" surrounds the solenoid 31in spaced relation. The solenoid is wound on a cylindrical non-magneticframe 36, as of 0.125" thick copper with an outside diameter of 9.250".The yoke 35 includes a pair of end plates 37 and 38, as of 0.750" thicksoft iron. The plates 37 and 38 are each formed by a pair of concentricring portions. The outer ring portions are fixedly secured to thecylindrical yoke portion 35, whereas, the inner ring portions form thepole pieces 37 and 38 of a linear X-band amplifier tube 39.

The amplifier tube 39 includes an evacuated envelope having a main bodyportion 41 closed at one end by an electron gun assembly 42 and closedat the other end by a beam collector structure 43. The main body portion41 includes a microwave interaction circuit for interaction with a beamof electrons to produce an amplified output signal. The interactioncircuit comprises 6 klystron type driver cavity resonators 44 which arestagger tuned over a band of interest and a three-coupled cavityextended interaction output cavity resonator 45. Signals to be amplifiedare coupled into the first resonator 44' via a waveguide, not shown. Thesecond resonator 44" is heavily loaded to an external resistive load viaa second waveguide 46. The amplified output signal saving aninstantaneous bandwidth of 4% at X-band and an average power of kw. iscoupled from the output resonator 45 via a third Waveguide, not shown.The waveguides are brought out of the tube 39 through three peripherallyspaced holes 47 in the collector pole piece 38, see FIGURE 7.

The cathode pole piece 37' includes a reentrant pole structure formed bya counter bore 51 (see FIGURE 8) in the plate 37'. The bore 51 defines amouth portion 52 of the reentrant pole structure. The bore 51 terminatesshort of passing through the plate 37'. A second bore 53 which iscoaxial of the first bore 51 and of lesser diameter defines a throatportion 54 of the reentrant pole structure. A concave cathode emitter 55is disposed on the axis of the tube 39 with the emitting surfaceintersecting the centerline of the tube 39 at a point 56 which isapproximately at the entrance plane 57 of the mouth 52. A cylindricalbeam focus electrode 58 surrounds the cathode emitter 55 forelectrostatically focusing the beam. A modulating anode electrode 59surrounds the beam path within the mouth 52 of the reentrant polestructure. The modulating anode electrode includes a tubular portion 61,as of copper, which extends into the throat 54' of the reentrant polepiece 37.

The pole piece 37' i sealed to a portion of the main body 41, as ofcopper. The main body 41 includes a first axial bore 62 of the samediameter as the throat 54 of the pole 37' and a second bore 63 formingthe beam hole of the main anode. The pole piece 37 and main body 41 areoperated at main anode potential which is typically ground potential.The cathode 55 and focus electrode 58 are operated negative with respectto the anode as of, for example, 37 kv. The modulating anode 53 isinsulated via insulators, not shown, from both the cathode 55 and anode41 and is pulsed between anode and cathode potential for pulsing thebeam current, as of 7.2 amps.

The mouth and throat portions 52 and 54 of the reentrant pole piece 37'are dimensioned relative to the cathode emitter 55 such that all theflux contained within the beam circumference at a point of its minimumdiameter within the main body 41 also threads through the cathodeemitter surface. This is achieved in the magnetic focusing structure ofthe present invention by positioning the cathode emitter 55 at theentrance plane 57 of the mouth 52 of the reentrant pole structure 37'and dimensioning and shaping the mouth 52 and throat 54- portions sothat the magnetic flux leaking through the throat 54 from the mainfocusing field will match the convergent electron trajectories in thegun region that the electrons would follow under electrostatic forcesalone. The dimensions for such a magnet circuit and electron gun designare shown in FIGURE 8 with both axial and radial scales provide for a50:1 area convergence, l.0 10* beam perveance, 37 kv.-7.2 amp beam, andemploying a main focusing magnetic field H of 3 kg. which is 2.5 timesthe Brillouin field intensity. The variation of the beam focusingmagnetic field on the beam axis is shown by dotted line 71 of FIGURE 8.The magnetic gap between the poles 37' and 38' is approximately 7.75long.

In general, for magnetic circuits of the present invention, point 56 ofthe cathode emitter 55- falls within 0.5 times the radius R of thecathode from the entrance lane 57 of the mouth 52 of the reentrant polestructure 37'. Also, the larger bore 51 has a radius falling between 2and 4 times the radius R of the cathode 55. The length of the mouth 52falls within 2 and 3 times the radius R of the cathode emitter 55. Thethroat 54 has a radius falling between 0.75 and 2 times the radius ofthe cathode and a length less than 1 radius of the cathode. In addition,the magnetic field H0 in the main field region in the gap between thepole pieces 37' and 38' peaks near the poles and dips to a minimum valuewhich is within of the peak intensity to prevent beam blow up.

In the reentrant pole design of the present invention, a preponderanceof the leakage field used for converging the beam in the gun regionterminates on the inside wall of the bore 51. In this region and in theannular plate-like region of the pole 37', the soft iron members arerelatively thick to prevent magnetic saturation effects. As aconsequence, the magnetic field may be varied over a relatively widerange as from 1800 gauss to 4000 gauss as the beam voltage is changedwithout disturbing the proper shape of the beam focusing field.

Referring now to FIGURE 9, there is shown an alternative electron gunand reentrant pole structure. The apparatus is essentially identical tothe structure of FIGURE 8 except that the modulating anode 59 has beeneliminated and replaced by the main anode 41 which fills the throatportion 54 of the reentrant pole structure 37'. The gun of FIGURE 9 hasa perveance of 1.0 10- the main magnetic field H has a value of 4 kg.,the beam voltage is 85 kv. with a 1" diameter cathode 55 and a beam areaconvergence of 40:1. The tube using the gun of FIGURE 9 has anefliciency of 45% at X-band. The beam interception is about 0.05% andthe drift tube filling factor is about 70%. The beam power is 2.1megawatts continuous.

Since many changes could be 'made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the ac companyingdrawings shall be interpreted as illustrative and not in a limitingsense.

What is claimed is:

1. In a microwave tube apparatus; means for forming and projecting abeam of electrons over an elongated beam path; means at the terminal endof the beam path for collecting and dissipating the energy of theelectrons; means forming a microwave circuit disposed along the beampath between said beam-forming means and said beam-collecting means forinteraction with the beam to produce an output microwave signal; meansforming a magnetic circuit for focusing the beam over its path be tweensaid beam-forming means and said beam-collecting means; said magneticcircuit including, a solenoid coaxially disposed of the beam path, amagnetic yoke made of magnetically permeable material surrounding saidsolenoid and having a pair of centrally apertured pole piece structuresthrough which the beam passes, one of said pole pieces forming thecathode pole piece and being disposed at the upstream end of the beampath, the improvement wherein; said cathode pole piece is reentranthaving a mouth portion and a throat portion; said beam forming meansincluding a cathode emitter disposed at the mouth portion of saidreentrant pole piece structure for emitting the beam of electronsthrough said throat portion ofsaid cathode pole piece structure; saidmouth and throat portions of said pole piece being shaped so that thebeam-focusing magnetic flux lines in said throat and mouth portions andwhich pass through said cathode emitter are converged to across-sectional area near said throat portion which is less than of thecross-sectional area of the beam at said cathode emitter, and whereinsaid mouth portion of said reentrant cathode pole structure is formed byan axially-directed counter bore in said pole piece, said bore having aradius of between 2 and 4 times the radius of said cathode emitter, andsaid bore having an axial length on the beam axis of between 2 and 3times the radius of said cathode emitter.

2. The apparatus of claim 1 including a modulating anode electrodestructure coaxially disposed of the beam path within said mouth portionof said cathode pole structure, said modulating anode electrode having atubular extension projecting axially of the beam path into the throatportion of said pole structure.

3. The apparatus of claim 1 wherein said throat portion of saidreentrant cathode pole structure is defined by a second axial bore insaid pole structure of lesser diameter than and coaxial with said firstbore, and wherein said second bore has a radius between 0.75 and 2 timesthe radius of said cathode emitter and a length less than 1 radius ofsaid cathode emitter.

4. The apparatus of claim 1 wherein the emitting surface of said cathodeemitter intersects the centerline of the beam at a point which is lessthan one-half of the radius of said cathode emitter from the entranceplane of said mouth portion of said reentrant cathode pole structure.

5. The apparatus of claim 1 wherein said magnetic circuit means producesa region of axially-directed beam focusing magnetic field which reachespeak amplitudes near said pair of pole structures and which sags to aminimum amplitude between said peaks, and wherein said minimum amplitudeis within 10% of the highest peak amplitude, whereby beam blow up isminimized.

References Cited UNITED STATES PATENTS 2,936,394 5/1960 Brewer 313-84 X2,966,609 12/ 1960 Turner 31384 X 3,255,370 6/1966 Geppert 3153.5 X

HERMAN KARL SAALBACH, Primary Examiner S. CHATMON, 1a., AssistantExaminer US. Cl. X.R.

